This proposal is predicated on the hypothesis first proposed by B. Pierce that "a determined stem cell required for tissue renewal is the cell of origin for carcinomas (3)." We propose that the development of new, improved preclinical mouse models for breast cancer may depend on the ability to target specific genetic events to stem/progenitor cells, and that this approach may overcome some of the key limitations of currently available mouse models of breast cancer. Furthermore, improved mouse and human models for studying the early steps in the progression of premalignant disease are required. The proposal is based upon several novel observations made during the previous funding period: 1) the role of p53 in hormone-dependent mammary gland progression, 2) the role of altered p53 in senescence, 3) the generation of a mouse Li-Fraumeni model carrying a gain-of-function p53 mutation, 4) the identification of stem/progenitor cell markers in the mammary gland and existing mouse breast cancer models 5) the development of methods to conditionally, and in some cases reversibly activate oncogenes, or delete tumor suppressor genes. The group of ten co-investigators from four institutions provides the necessary and complementary expertise required to generate and characterize the proposed new mouse models of cancer. The following specific aims are proposed: 1) To generate new mouse models capable of inducibly expressing molecules in Bcrp1+ mammary side-population (SP) cells, and to determine the impact of oncogenic signaling pathways on SP ceils, and the requirement of these cells for mammary tumorigenesis. 2) To develop an inducible mouse model to functionally assess oncogenesis in mammary gland Sca-1+ progenitor cells by a knock-in of the reverse tetracycline-responsive transactivator (rtTA) and the avian leukosis virus TVA receptor into the Sca-1 locus. 3) To identify cell lineage markers for stem and progenitor cells in both the normal mammary gland and the p53 null Balb/c model of breast cancer progression in order to understand the role of stem cells in breast cancer progression and in the etiology of hormone-dependent and independent breast cancer. To understand the role of p53 in mammary stem cell renewal and senescence. 4) To develop a human xenograft model from unfolded Iobules to study early breast cancer progression for comparison with the progression models in the mouse.